How are Centrosymmetric and Noncentrosymmetric Structures

Feb 16, 2010 - Department of Civil Engineering and Geological Sciences and ... to explore how uranyl borates can use similar structural motifs to yiel...
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2948 Inorg. Chem. 2010, 49, 2948–2953 DOI: 10.1021/ic902480n

How are Centrosymmetric and Noncentrosymmetric Structures Achieved in Uranyl Borates? Shuao Wang,† Evgeny V. Alekseev,*,‡,† Jared T. Stritzinger,† Wulf Depmeier,‡ and Thomas E. Albrecht-Schmitt*,† †

Department of Civil Engineering and Geological Sciences and Department of Chemistry and Biochemistry, 156 Fitzpatrick Hall, University of Notre Dame, Notre Dame, Indiana 46556, and ‡ Institut f€ ur Geowissenschaften, Universit€ at zu Kiel, 24118 Kiel, Germany Received December 14, 2009

Four uranyl borates, UO2B2O4 (UBO-1), R-(UO2)2[B9O14(OH)4] (UBO-2), β-(UO2)2[B9O14(OH)4] (UBO-3), and (UO2)2[B13O20(OH)3] 3 1.25H2O (UBO-4), have been prepared from boric acid fluxes at 190 C. UBO-3 and UBO-4 are centrosymmetric, whereas UBO-1 and UBO-2 are noncentrosymmetric (chiral and polar). These uranyl borates possess layered structures constructed from UO8 hexagonal bipyramids, BO3 triangles, and BO4 tetrahedra. In the case of UBO-4, clusters of BO3 triangles link the layers together to form open slabs with a thickness of almost 2 nm. The ability of uranyl borates to use very similar layers to yield both centrosymmetric and noncentrosymmetric layers is detailed in this work.

Introduction Uranyl borates represent a fascinating group of zero-, two-, and three-dimensional materials. A variety of synthetic methods have been employed to prepare these compounds, and the conditions under which these solids form leads to their subdivision into three categories. The first group is a series of chiefly centrosymmetric borates that are primarily constructed from UO6 tetragonal bipyramids and/or UO7 pentagonal bipyramids and BO3 triangles.1-6 These compounds are derived from high-temperature B2O3 melts. The second category has only a single representative, K6[UO2{B16O24(OH)8}] 3 12H2O, and this compound consists of isolated clusters composed of a cyclic polyborate surrounding a uranyl core.7 It forms via the slow evaporation of aqueous solutions contain UO22þ and borate at room temperature. The third group is prepared via the use of boric acid fluxes at relatively low temperatures (ca. 200 C). These compounds blend features found in both previous categories in that they contain polyborate layers and frameworks that are built from both triangular BO3 and tetrahedral BO4 units *To whom correspondence should be addressed. E-mail: talbrec1@ nd.edu. (1) Gasperin, M. Acta Crystallogr. 1987, C43, 1247. (2) Gasperin, M. Acta Crystallogr. 1987, C43, 2031. (3) Gasperin, M. Acta Crystallogr. 1987, C43, 2264. (4) Gasperin, M. Acta Crystallogr. 1988, C44, 415. (5) Gasperin, M. Acta Crystallogr. 1989, C45, 981. (6) Gasperin, M. Acta Crystallogr. 1990, C46, 372. (7) Behm, H. Acta Crystallogr., Sect. C 1985, 41, 642. (8) Wang, S.; Alekseev, E. V.; Ling, J.; Skanthakumar, S.; Soderholm, L.; Depmeier, W.; Albrecht-Schmitt, T. E. Angew. Chem., Int. Ed. 2010, 49, 1263. (9) Wang, S.; Alekseev, E. V.; Ling, J.; Liu, G.; Depmeier, W.; AlbrechtSchmitt, T. E. Chem. Mater. 2010, [Online].

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that surround uranyl cations.8,9 All of these compounds contain UO8 hexagonal bipyramids. As a general class of compounds, borates are known from their applications in optics, especially for their large nonlinear optical responses in the deep UV, where they are much more transparent than most oxoanion compounds.10-12 In order for a compound to display a second-order nonlinear optical (NLO) response, it must lack a center of symmetry. While borates are known for achieving this, a survey of the space groups of borates reveals that noncentrosymmetric structures are not common, making the preparation of new borate NLO materials challenging.13,14 We have recently demonstrated that both sodium and thallium uranyl borates are either chiral or polar. Eight such compounds have been reported by us.8,9 These results are surprising because the presence of the uranyl cation has a propensity for yielding centrosymmetric structures because of the highly symmetric nature of UO22þ. Nevertheless, all of the uranyl borates that we have prepared that contain additional monovalent cations are noncentrosymmetric.9 However, we know that centrosymmetric structures are possible in actinyl borates from the structures of the neptunyl borates, K4(NpO2)6.73B20O36(OH)2 and Ba2(NpO2)6.59B20O36(OH)2 3 H2O.8 (10) Mori, Y.; Yap, Y. K.; Kamimura, T.; Yoshimura, M.; Sasaki, T. Opt. Mater. 2002, 19, 1. (11) Chen, C.; Wu, B.; Jiang, A.; You, G. Sci. Sin. 1984, B7, 598. (12) Mori, Y.; Sasaki, T. Bull. Mater. Sci. 1999, 22, 399. (13) Burns, P. C.; Grice, J. D.; Hawthorne, F. C. Can. Mineral. 1995, 33, 1131. (14) Burns, P. C.; Grice, J. D.; Hawthorne, F. C. Can. Mineral. 1999, 37, 731.

r 2010 American Chemical Society

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Table 1. Crystallographic Data for UO2B2O4 (UBO-1), R-(UO2)2[B9O14(OH)4] (UBO-2), β-(UO2)2[B9O14(OH)4] (UBO-3), and (UO2)2[B13O20(OH)3] 3 1.25H2O (UBO-4) compound

UBO-1

UBO-2

UBO-4

925.35 yellow-green, block P1 6.4169(5) 6.4494(5) 11.0177(12) 97.604(1) 92.365(1) 119.475(1) 390.46(6) 1 296(2) 0.71073 28.62 3.935 208.41 0.0200 0.0447

1068.59 yellow, needle P1 6.4732(5) 10.7981(8) 19.6653(15) 85.711(1) 82.660(1) 89.287(1) 1359.48(18) 1 293(2) 0.71073 28.67 2.610 120.05 0.0558 0.1370

)

)

mass 355.65 925.35 color and habit yellow, block yellow-green, plate space group C2 P21 a (A˚) 10.4607(9) 6.4534(4) b (A˚) 4.1863(4) 21.2535(12) c (A˚) 5.6251(5) 6.4548(4) R (deg) 90 90 β (deg) 109.769(1) 119.874(1) γ (deg) 90 90 231.81(4) 767.68(8) V (A˚3) Z 2 2 T (K) 293(2) 100(2) λ (A˚) 0.71073 0.71073 maximum 2θ 28.68 28.83 5.095 4.003 F calcd (g cm-3) μ (Mo KR) 349.49 212.00 0.0139 0.0316 R(F) for Fo2 > 2σ(Fo2)a 0.0332 0.0725 Rw(Fo2)b P P P a b 2 2 2 2 P 4 1/2 R(F) = Fo| - |Fc / |Fo|. R(Fo) = [ w(Fo - Fc ) / w(Fo)] .

UBO-3

We have recently prepared pure uranyl borates from boric acid fluxes that lack additional cations. Two of these compounds are centrosymmetric; two are noncentrosymmetric. These compounds are UO2B2O4 (UBO-1), R-(UO2)2[B9O14(OH)4] (UBO-2), β-(UO2)2[B9O14(OH)4] (UBO-3), and (UO2)2[B13O20(OH)3] 3 1.25H2O (UBO-4). These uranyl borates are all new, and possess fascinating structures and more importantly provide us with an opportunity to explore how uranyl borates can use similar structural motifs to yield both centrosymmetric and noncentrosymmtric structures, which is the purpose of this article. Experimental Section Syntheses. UO2(NO3)2 3 6H2O (98%, International Bio-Analytical Industries), boric acid (99.99%, Alfa-Aesar), ammonium chloride (99.6%, Fisher), and cesium nitrate (99.8%, AlfaAesar) were used as received. All the reactions were run in PTFE-lined Parr 4749 autoclaves with a 23 mL internal volume. Distilled and Millipore filtered water with resistance of 18.2 MΩ 3 cm was used in all reactions. Caution! While the UO2(NO3)2 3 6H2O used in this study contained depleted U, there is really not much difference between depleted uranium and natural abundance uranium, and standard precautions for handling radioactive materials should be followed at all times. There are very old sources of uranyl nitrate that may not be depleted, and enhanced care is warranted for these samples. UO2B2O4 (UBO-1). UO2(NO3)2 3 6H2O (0.5000 g, 1 mmol), boric acid (0.4944 g, 8 mmol), and water (50 μL) were loaded into a 23 mL autoclave. The autoclave was sealed and heated to 190 C in a box furnace for 3 days. The autoclave was then cooled down to room temperature at a rate of 5 C/h. The products were washed with boiling water to remove excess boric acid, followed by rinsing with methanol. Crystals in the form of needles and prisms with light yellow coloration were collected as a pure phase for UO2B2O4 (UBO-1). Yield: 123 mg (35% based on uranium). r-(UO2)2[B9O14(OH)4] and β-(UO2)2[B9O14(OH)4] (UBO-2 and UBO-3). UO2(NO3)2 3 6H2O (0.5000 g, 1 mmol), boric acid (1.3586 g, 22 mmol), ammonium chloride (0.0535 g, 1 mmol), and water (50 μL) were loaded into a 23 mL autoclave. The autoclave was sealed and heated to 190 C in a box furnace for 3 days. The autoclave was then cooled down to room temperature at a rate of 5 C/h. The products were washed with boiling water to remove excess boric acid, followed by rinsing with methanol. Crystals in the form of plates and blocks with light

yellow-green coloration were collected for R-(UO2)2[B9O14(OH)4] and β-(UO2)2[B9O14(OH)4] (UBO-2 and UBO-3). UBO-1 is also present as a minor product. (UO2)2[B13O20(OH)3] 3 1.25H2O (UBO-4). UO2(NO3)2 3 6H2O (0.5000 g, 1 mmol), boric acid (0.4944 g, 8 mmol), cesium nitrate (0.3898 g, 2 mmol), and water (90 μL) were loaded into a 23 mL autoclave. The autoclave was sealed and heated to 190 C in a box furnace for 3 days. The autoclave was then cooled down to room temperature at a rate of 5 C/h. The products were washed with boiling water to remove excess boric acid, followed by rinsing with methanol. Crystals in the form of needles with light yellow coloration were collected as a pure phase for (UO2)2[B13O20(OH)3] 3 1.25H2O (UBO-4). Yield: 144 mg (27% based on uranium). Crystallographic Studies. Single crystals of all four UBO phases were mounted on glass fibers and optically aligned on a Bruker APEXII CCD X-ray diffractometer or a Bruker APEXII Quazar X-ray diffractometer using a digital camera. Initial intensity measurements were either performed using an IμS X-ray source, a 30 W microfocused sealed tube (Mo KR, λ = 0.71073 A˚) with high-brilliance and high-performance focusing Quazar multilayer optics, or a standard sealed tube with a monocapillary collimator. Standard APEXII software was used for determination of the unit cells and data collection control. The intensities of reflections of a sphere were collected by a combination of four sets of exposures (frames). Each set had a different j angle for the crystal, and each exposure covered a range of 0.5 in ω. A total of 1464 frames were collected with an exposure time per frame of 10 to 50 s, depending on the crystal. The SAINT software was used for data integration including Lorentz and polarization corrections. Semiempirical absorption corrections were applied using the program SADABS.15 Selected data and crystallographic information are listed in Table 1. Atomic coordinates, bond distances, and additional structural information are provided in the Supporting Information (CIF). Powder X-Ray Diffraction. Powder X-ray diffraction patterns of products of pure UBO-1 and relatively pure UBO-4 were collected on a Scintag θ-θ diffractometer equipped with a diffracted-beamed monochromatic set for Cu KR (λ = 1.54056 A˚) radiation at room temperature in the angular range from 5 to 80 (2θ) with a scanning step width of 0.05 and a fixed counting time of 1 s/step. The collected patterns were (15) Sheldrick, G. M. SADABS 2001, Program for absorption correction using SMART CCD based on the method of Blessing: Blessing, R. H. Acta Crystallogr. 1995, A51, 33.

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Figure 1. (a) A view of the UBO-1 crystal structure. (b) The structure of the uranyl borate sheets in UBO-1. (c) The relative positions of the layers in R-UBO-1. (d) The relative positions of the layers in β-UBO-1.

compared with those calculated from single crystal data using ATOMS (see the Supporting Information). Infrared Spectroscopy. Infrared spectra were obtained from a single crystal of UBO-4 using a SensIR technology IlluminatIR FT-IR microspectrometer. A single crystal of UBO-4 was placed on a glass slide, and the spectrum was collected with a diamond ATR objective. Each spectrum was taken from 650 to 4000 cm-1 with a beam aperture of 100 mm. Infrared spectra are shown in the Supporting Information. Fluorescence Spectroscopy. Fluorescence data were acquired on all four compounds from single crystals using a Craic Technologies UV-vis-NIR microspectrophotometer with a fluorescence attachment. Excitation was achieved using 365 nm light from a mercury lamp. The fluorescence spectra are provided in the Supporting Information.

Results and Discussion Synthesis. Boric acid becomes a reactive flux when the reaction temperature is raised above its melting point (170.9 C). It is an ideal flux for prepared novel actinide borates.8,9,16 UO2B2O4 (UBO-1) is observed in reactions that contain high U/B ratios when alkali metal or alkaline-earth metal cations are also present. It can be made as a pure phase when the U/B ratio is higher than 1:8 when additional cations are not present. R-(UO2)2[B9O14(OH)4] (UBO-2) and β-(UO2)2[B9O14(OH)4] (UBO-3) form simultaneously in the presence of ammonium chloride. We were surprised to find that NH4þ does not directly incorporate into uranyl borates in the same way that alkali metal cations and Tlþ do.9 Although its presence plays a definite role in crystallization. Similarly, (UO2)2[B13O20(OH)3] 3 1.25H2O (UBO-4) can only be isolated in the presence of trace amounts of Csþ. In contrast, cesium uranyl borates can be prepared, but these are the subject of a subsequent work. The yields reported in this work are with three day reaction times. We know that yields can be improved with substantially longer heating times (>7 days). (16) Wang, S.; Alekseev, E. V.; Diwu, J.; Casey, W. H.; Phillips, B. L.; Depmeier, W.; Albrecht-Schmitt, T. E. Angew. Chem., Int. Ed. 2010, 49, 1057.

Wang et al.

Figure 2. (a) A depiction of the UBO-2 crystal structure. (b) A general view of the UBO-3 crystal structure. (c) The structure of borate sheets in UBO-2 and UBO-3. (d) The three topologies of uranyl borate layers realized in UBO-2 and UBO-3.

Structure Descriptions. The structures of all synthesized phases are based on single (UBO-1) or double layers (UBO-2, UBO-3, and UBO-4). The simplest sheets are realized in the crystal structure of UBO-1 (Figure 1). They are based on BO3 triangles linked by vertices into (BO2)1¥ 1D zigzag-like chains (Figure 1a). These chains are united into 2D layers by edge-sharing with UO8 hexagonal bipyramids that also form a 1D chain with a (UO6)1¥ composition. The resulting layers pack into a regular 3D structure (Figure 1a) with weak connectivity (mostly via van der Waals forces). UBO-1 is a second modification of a previously known uranyl borate with the UO2B2O4 composition.2 However, this phase was synthesized using a high-temperature solid-state reaction at 1150 C. The temperature of preparation and symmetry are higher for the modification obtained by Gasperin,2 and we are referring to this modification as β-UO2B2O4 in this work. Both modifications are based on identical 2D sheets (Figure 1a), but the orientation of them is different in R-UO2B2O4 versus β-UO2B2O4. In Figure 1c, it is demonstrated that the (BO2)1¥ chains are arranged in identical positions in sequential layers in R-UO2B2O4. However, as shown in Figure 1d, the (BO2)1¥ chains in β-UO2B2O4 are arranged differently as the sheets are stacked. The layered structure of β-UO2B2O4 is identical but oriented differently;the neighboring sheets are rotated by 180 to each other. This rotation is produced by doubling the c parameter in β modification. This type of polymorphism can be referred to as rotational polytypism as described early on for micas.17 The UBO-2 and UBO-3 compounds have identical chemical compositions but crystallize in different space groups. We named UBO-2 as R-(UO2)2[B9O14(OH)4] and UBO-3 as the β modification. Both phases have layered structures but are very different from UBO-1 in that they possess double layers that are based on two sublayers (Figure 2a and b). These sublayers consist of 2D polyborate sheets that incorporate the nearly linear uranyl groups, UO22þ, creating UO8 hexagonal bipyramids. The borate sheets are based on BO4 tetrahedra packed in a (17) Takeuchi, Y.; Sadanaga, R. Acta Crystallogr. 1959, 12, 945.

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Figure 4. A schematic representation of F-type layer construction using neptunyl and plutonyl borate sheets. (a) Neptunyl borate sheet structure. (b) Plutonyl borate sheet structure. (c) Resulting sheet with F-type topology.

Figure 3. (a) An illustration of the UBO-4 crystal structure. (b) The structure of uranyl borate sheets in UBO-4. (c and d) The two topologies of uranyl borate sheets realized in UBO-4.

triangular manner (shown in green color in Figure 2c) and BO3 triangles (shown in red in Figure 2c), which combine these triangular groups into a 2D network. These 2D sheets are united by perpendicular BO3 groups into double layers. The resulting layers are packed with a slight shift into a 3D structure. The layers are connected by strong hydrogen bonds with a bond length of 2.42 A˚ between the O(15) and O(22) oxygen atoms. The polymorphism in UBO-2 and UBO-3 is of a different nature than the polymorphism in UO2B2O4. In general, we can follow this in the same way that we used to describe the Na-uranyl borates and Tl-uranyl borates.9 In this work, we illustrated several different types of borate sheets. In the structures of UBO-2 and UBO-3, three of these sheets, A, A0 , and B (Figure 2d), occur. In the R phase, there are alternating enantiomorphic A0 and B layers, and the resulting structure is chiral and polar. In the β phase, only similar A and A0 layers are found, and they create a symmetry center in UBO-3. It is unusual that, in all of the uranyl borate systems (U-B, Na-U-B, and Tl-U-B) that we have prepared, polymorphism is observed. These include R,β-UO2B2O4, R,β-(UO2)2[B9O14(OH)4] (this work), R,β-Na[(UO2)2B10O15(OH)5)], and R,β-Tl2[(UO2)2B11O18(OH)3].9 Such polymorphic richness is very surprising for uranyl chemistry and makes the uranyl borate system unique within groups of binary and ternary uranyl compounds. The layers in UBO-4 are also doubled as found in structures of UBO-2 and UBO-3 (Figure 3a), but there are substantial differences as well because the layers in UBO-4 have a nanoscale thickness of ∼19 A˚. The thick slabs are the result of very unusual linkers and additional

triangular BO3 groups on both sides (top and bottom) of the layers, as shown in Figure 3a. The linkers in doubled layers consist of one BO3 group connected with two BO4 tetrahedra within the sublayers, and a supertriangle that is the second part of the linker. This supertriangle consists of three flat BO3 groups combined by corners within the supertriangle, and with the BO4 and BO3 groups from layers (Figure 3a). These supertriangles are perpendicular to the sublayer planes, and their position produces large free space within the layers that is occupied by disordered water molecules. The sublayers in UBO-4 are slightly corrugated, and as a result of this, the uranyl groups are not strongly perpendicular to the plane of the layer. The topology of the sublayers is different from all previously known types in actinide borate sheets.1-9,18 The layers are based on two types of groups that we can call “building blocks” and “linkers.” The building blocks consist of two tetrahedral BO4 groups (directed on one side) and one BO3 triangle. All of these units share corners, and the projection of it onto the sublayer’s plane is triangularlike. The building blocks are united by two sorts of “linkers”; these are BO3 triangles and BO4 tetrahedra. These groups are linked together into building blocks that yield a regular 2D network, as shown in Figure 3b, which we call F-type (Figure 3c,d) layers. In the structure of UBO-4, there are F type and symmetrically dependent F0 layers (for symmetry details, see ref 9). Both sheets take place in one double layer. The general topology of these sheets is new, but it can be represented as the superposition of the sheets realized in neptunyl and plutonyl borates.8 The scheme of this construction is shown in Figure 4. In Figure 4a and b, fragments of the sheets in neptunyl and plutonyl borates are shown, respectively. The dashed line shows selected 1D fragments separated from both structures. These are fragments 1 and 2. Fragment 2 can be transformed to 20 by reflection in the sheet plane (identical to the reverse view). In Figure 4c, it is demonstrated that the results of the 1 and 20 1D fragments are combined into the 2D structure where these fragments are regularly alternated. The resulting sheet is (18) Burns, P. C. Can. Mineral. 2005, 43, 1839.

2952 Inorganic Chemistry, Vol. 49, No. 6, 2010 identical with F (or F0 after symmetrical transformation). In the same manner, we can create the E-type sheets in uranyl borates that are found in R-Tl2[(UO2)2B11O18(OH)3]. However, the E layer construction found the neptunyl borate sheets are the result of only one BO4 tetrahedral linker.8 The layers in UBO-4 diverge sharply from other uranyl borates in other ways as well. There are two uranium sites in the layers, both of which are surrounded by nine borate groups. However, one site is surrounded by four BO3 units and five BO4 units. The other site is surrounded by three BO3 units and six BO4 units, which are also different from the typical uranyl borate coordination environment because these BO3 units are not separated by two BO4 units from each other. Interestingly, this coordination environment is exactly the same as the neptunyl borates.8 The primary difference between UBO-4 and K4(NpO2)6.73B20O36(OH)2 and Ba2(NpO2)6.59B20O36(OH)2 3 H2O is that the neptunyl borates have additional neptunyl units in the voids that are held in place primarily through cation-cation interactions,8 which are exceedingly rare in U(VI) compounds. The channels that extend within the layers are large at 12.002  5.224 A˚. In UBO-4, the channels are filled with disordered water. Finally, there is a strong stoichiometric dependence from the reactions that is reflected in the B/U ratio found in the crystals: B/U = 2 (UBO-1), B/U = 4.5 (UBO-2, UBO-3), and B/U = 6.5 (UBO-4). The packing principles show only small variations, and in all phases the layers pack to layers, and we observe only 2D structures. However, as the amount of boron is increased, the structures became more 3D-like. UBO-2 and UBO-3 (B/U = 4.5) have doubled layers, but very little free space between the layers. However, UBO-4 (B/U = 6.5) possesses a 3D framework with large voids within and between the layers that are occupied by water molecules. Such structural behavior of the UO22þ-B2O3 system is quite unusual because in other similar systems dimensionality decreased with increasing of the X/U ratios (X = Mo, W, P, As, etc.).19 Symmetry Features. UBO-1 and UBO-2 are noncentrosymmetric, whereas UBO-3 and UBO-4 possess a center of symmetry. The analysis of the UBO-1 crystal structure gives a simple understanding of the noncentrosymmetry of this phase. In Figure 1, we can see that the layers alternate and shift. This occurs to maximize close packing. The layers do not have an internal center of symmetry, and the packing type does not produce one as well. Understanding the lack of a center of symmetry in UBO-2, UBO-3, and UBO-4 is more difficult because of the high complexity of these crystal structures. We described the layer types (A/A0 , B, and F/F0 ) in the previous section. The A and B layer types are enantiomorphic (we can name A “right” and B “left”, see Figure 2c). Detailed analysis of the UBO-2 crystal structure demonstrates that the A0 sheets are rotated 30 in comparison to the neighboring B sheets in the plane of the doubled layers (Figure 2a). This rotation is necessary for layers to be linked by the BO3 triangles, but it makes a center of symmetry impossible within the double layers. The opposite situation occurs in the structure of UBO-3 (19) Alekseev, E. V.; Krivovichev, S. V.; Armbruster, T.; Depmeier, W.; Suleimanov, E. V.; Chuprunov, E. V.; Golubev, A. V. Z. Anorg. Allgem. Chem. 2007, 633, 1979.

Wang et al. (centrosymmetric structure), and there are A and A0 sheets. The A0 sheets are rotated 180 þ 1/4b shift in comparison with A layers in the same double layers. This rotation is also necessary for the linking of the A and A0 sheets by the BO3 triangles. This configuration is centrosymmetric because of two equivalent BO3 linkers (the boron atoms are in split positions). In the structure of UBO-4, we can see the same situation. The F and F0 sheets are united in one double layer. The neighboring F sheets also rotated 180 þ 1/2a þ 1/2b shift. This is the same situation that occurs in the structure of UBO-3, and it also yields a center of symmetry. A comparison of pure uranyl borates with Na and Tl phases that also possess enantiomorphic sheets yields some interesting results.9 All eight structures investigated in Na and Tl systems are noncentrosymmetric. Six of them are based on the same sheet alternation as UBO-2. The internal rotation of the sheets in these structures is 120, which is related by 30 in UBO-2. There are other structures that are also based on one type of sheet, but centers of symmetry do not occur here because the sheets are rotated by 120 in NaUBO-4 (as in UBO-2) as well as close packing of the same layers (E-type in TlUBO-1) in UBO-1. Thus, we can say that the achievement of centrosymmetric and noncentrosymmetric structures strongly depends on sheet topologies and the disposition of the sheets as they pack. The relative positions of the sheets are dependent on the chemical compositions. When excess boron is present in the syntheses, doubled layers normally result with additional BO3 (or B2O5 and B4O8) groups that link the layers together. As such, we can try to regulate symmetry by adjusting reaction stoichiometry. Conclusions Of the 12 uranyl borates that we have now reported, 10 are noncentrosymmetric, the majority of which adopt polar space groups. Of the seven previously reported uranyl borates derived from either B2O3 melts or room temperature crystallization, only one is noncentrosymmetric. What makes this result even more surprising is that in some of high-temperature phases the uranium coordination environment is a UO7 pentagonal bipyramid. This building unit should show a greater propensity for acentric structures than the UO8 hexagonal bipyramids that we have observed because in the former unit the uranium atoms cannot be placed on centers of inversion (without disorder), whereas in the latter a center of symmetry is easily invoked. Of equal importance is the fact that similar layers that are capable of yielding noncentrosymmetric structures can also yield centrosymmetric structures as found in UBO-2 and UBO-3. In the future, we will disclose other families of uranyl borates that will highlight these themes. All of these actinide borates provide useful models for the crystallized portions of vitrified nuclear waste.20-22 These are likely to be one of the only ways that we can glimpse into vitrified nuclear waste because a typical glass log with (20) Ramsey, W. G. Glass as a Waste Form and Vitrification Technology: Summary of an International Workshop; National Academy of Science: Washington, 1996. (21) (a) Krivovichev, S. Minerals as Advanced Materials I; Springer: Heidelberg, Germany, 2008. (b) Farnan, I.; Cho, H.; Weber, W. J. Nature 2007, 445, 190. (c) Ewing, R. C. Nature 2007, 445, 161. (d) Ewing, R. C. Proc. Natl Acad. Sci. USA 1999, 96, 3432. (22) Meaker, T. F. Neptunium Immobilization and Recovery Using Phase Separated Glasses; Department of Energy: Washington, DC, 1996.

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high-level waste yields on the order of 200,000 Ci (i.e., lethal doses within seconds). Acknowledgment. We are grateful for support provided by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, Heavy Elements Program, U.S. Department of Energy, under Grants DE-FG02-01ER15187 and DE-FG02-01ER16026, and by Deutsche Forschungsgemeinschaft for support within the DE 412/30-2 research project. This material is based upon work supported as part of the Materials Science of Actinides, an Energy

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Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001089. The National Science Foundation also supported a portion of this work through the REU program in solid-state and materials chemistry (DMR). Supporting Information Available: X-ray crystallographic files in CIF format for UO2B2O4 (UBO-1), R-(UO2)2[B9O14(OH)4] (UBO-2), β-(UO2)2[B9O14(OH)4] (UBO-3), and (UO2)2[B13O20(OH)3] 3 1.25H2O (UBO-4). This material is available free of charge via the Internet at http://pubs.acs.org.